The present invention relates generally to an apparatus and method for hard rock sidewall coring of a borehole, and more particularly to a rotary sidewall coring tool that employs a direct drive mechanism, which operates at an enhanced efficiency, a coring bit control circuit, which provides for precise control of bit advancement, and a carousel core storing device that enables the storage of a large number of core samples.
Conventional tools for hard rock sidewall coring of a borehole employ complex drive mechanisms, which are not very efficient. Many of these systems also provide inadequate torque delivery at the coring bit making them incapable of delivering reliable core operation. In one such system, the drive mechanism comprises an electric motor coupled to a hydraulic pump, which in turn is coupled to a hydraulic motor, which drives the bit. There is a significant power loss in the hydraulic pump and hydraulic motor of such systems. This is because the down hole temperatures are very high, which lowers the viscosity of the hydraulic fluid in the hydraulic pump and motor, which in turn causes a significant amount of the hydraulic fluid to seep past the pistons in the hydraulic pump and motor, which results in a loss of power output by the pistons. Up to sixty percent (60%) of the efficiency of the hydraulic pump and motor can be lost through the drop in viscosity of the hydraulic fluid. Additional efficiency of such systems are lost because they employ a second hydraulic pump to drive the auxiliary devices, which is a drain on the power output of the electric motor. Thus, such systems can lose up to seventy percent (70%) of their efficiency. Hydraulic motors, therefore, have losses due to low volumetric efficiency (fluid loss) and mechanical efficiency (losses due to gears and bearings) which make their overall efficient less than ideal.
In another conventional system, the drive mechanism comprises an electric motor coupled to a hydraulic pump, which is in turn coupled to a hydraulic motor in turn coupled to a 90° transmission. This system has the same drawbacks of the previously described system, namely that there are significant loses due to the decrease in viscosity of the hydraulic fluid in the hydraulic motor. The drive mechanism in this system outputs a low speed and high torque to the bit. Because of its slow speed, this system takes longer than the other systems to remove each core sample. Thus, it requires more rig operation time, thereby making it more expensive to employ.
Furthermore, conventional tools for hard rock sidewall coring of a borehole employ limited feedback of operating conditions. While such devices have the ability to control the advancement of the core bit during coring, they do not have the ability to monitor in real time the torque of the bit. Since torque is a primary factor in determining the rate of penetration of the bit, conventional coring devices lack an important piece of information to prevent stalling of the bit during the coring operation. Rather, such devices infer the torque or RPM from the pressure response or motor current changes during the coring operation. However, because inferential readings are inherently inaccurate, conventional coring devices are susceptible to stalling.
Another disadvantage of conventional tools for hard rock sidewall coring of a borehole is that they have limited space in which to store the core samples. Accordingly, only a limited number of samples can be stored in such devices during a single run of the tool. In certain wells, therefore, the tool must be run down hole more than once to collect all of the desired core samples. A tool with larger core sample storage capacity is desirable.